Modern circuit protection for a single-ended (single source) three-phase, four-wire power distribution network is provided by static trip circuit interrupters, such as circuit breakers, equipped with three current transformers for separately sensing the phase currents flowing in each phase conductor. The current outputs of these transformers are separately applied to a static trip unit for processing to determine the existence of an overcurrent condition. If an overcurrent condition is found to exist, an overcurrent trip function is initiated leading to interruption of the phase conductors. If, in addition to overcurrent protection, ground fault protection is also desired, the phase current sensing transformer outputs are also individually applied, together with the current output of a neutral current sensing transformer, to some type of current differential or residual circuit. Should an overall imbalance in the currents flowing in the phase and neutral conductors of the network exist, as occasioned by ground fault current returning to the source via an external ground path, a ground fault signal proportional to the overall current imbalance is produced. This ground fault signal is processed by the static trip unit pursuant to initiating a ground fault trip function likewise leading to the opening of interrupter contacts and consequent interruption of at least the phase conductors. Static circuit interrupter trip units capable of affording integral overcurrent and ground fault protection are disclosed in U.S. Pat. No. 3,786,311 and commonly assigned, co-pending application Ser. No. 794,700, filed May 6, 1977.
For ground fault protection of double-ended (two sources) power distribution networks, the typical industry practice is to resort to separate zero sequence transformers for sensing current imbalances in the main phase and neutral conductors or buses for each source and in the interconnecting or tie phase and neutral buses. The signal outputs of these transformers are applied to separate ground fault relays operating to selectively shunt trip associated main and tie circuit interrupters pursuant to removing the faulted bus from the network, while continuing network power distribution via the remaining unfaulted bus. Examples of systems of this type for protecting multiple source distribution networks from ground faults are disclosed in U.S. Pat. Nos. 3,259,802, 3,558,981 and 3,949,272.
It is a principal object of the present invention to provide a ground fault protection system for multiple source power distribution networks which utilizes one or more circuit interrupters having static trip units operable to execute a ground fault trip function.
A further object is to provide a ground fault protection system of the above character which is constructed to distinguish between normal neutral loading, wherein the neutral currents take diverse return paths, and a ground fault condition.
An additional object is to provide a ground fault protection system of the above character which is capable of recognizing the full extent of a ground fault condition even though ground leakage current may return to the source feeding the fault via neutral conductors of the distribution network.
Yet another object of the present invention is to provide a ground fault protection system of the above character wherein the circuit interrupters are selectively tripped to remove the ground faulted bus from the network while continuing electrical service over the remaining unfaulted bus.
An additional object of the present invention is to provide a ground fault protection system of the above character which is inexpensive to implement and reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.